1. Technical Field of the Invention
The present invention relates generally to a sensor element of an oxygen sensor which may be employed in air-fuel ratio control of internal combustion engines, and more particularly to an improved internal structure of such a sensor element which is designed to produce a sensor output accurately at a quick response rate.
2. Background Art
There are used oxygen sensors for controlling the air-fuel ratio of a mixture supplied to an internal combustion engine of an automotive vehicle. This type of oxygen sensor is usually disposed in an exhaust system of the engine to measure the concentration of oxygen contained in exhaust gasses and includes a sensor element which consists essentially of a solid electrolyte body, a target gas electrode, and a reference gas electrode.
The target gas electrode is disposed within a gas chamber filled with a gas to be measured. The target gas electrode is covered with a porous protective layer made of MgO.Al2O3. The reference gas electrode is disposed in a reference gas chamber. The solid electrolyte body is usually made of an oxygen ion conductive material such as a zirconia-based ceramic material. The solid electrolyte body works to produce the electromotive force as a function of the concentration of oxygen contained in the gasses and outputs a sensor signal through the target gas electrode and the reference gas electrode.
In recent years, the emission regulations have been made more rigorous. In order to meet this requirement, the improvement of accuracy and durability of the above oxygen sensor is sought for enhancing the burning efficiency of the engine. The oxygen sensor element are, therefore, required to output a sensor signal accurately at a quick response rate over a wide temperature range. The oxygen sensor element is, as described above, designed as a concentration cell which produces the electromotive force as a function of a difference between concentrations of oxygen contained in gasses to which the target gas and reference gas electrodes are exposed, respectively. The target gas electrode and the reference gas electrode are each made of platinum exhibiting the catalysis so as to produce the electromotive force which changes greatly across the concentration of oxygen corresponding to the stoichiometric air-fuel ratio (i.e., an excess coefficient or air ratio xcex=1). Producing a sensor signal accurately at a quick response rate in this type of oxygen sensor element requires a decreased shift in xcex-point at which the electromotive force indicates the air ratio xcex=1.
In order to realize such an oxygen sensor element, Japanese Patent First Publication No. 2-1511755 (corresponding to U.S. Pat. No. 5,443,711) and Japanese Patent First Publication No. 1-203963 propose the formation of a catalytic layer on the target gas electrode for minimizing the shift in xcex-point. The catalytic layer is made of a carrier which is formed by a nonstoichiometric compound of a transition metal oxide such as TiO2 and has catalytic metal grains held therein. The oxygen sensor elements as taught in the above publications, however, lack the stability of operation over a wide environmental range and has a difficulty in decreasing the xcex-point sufficiently.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an oxygen sensor element which is designed to produce a sensor output accurately over a wide temperature range and a manufacturing method thereof.
According to one aspect of the invention, there is provided an oxygen sensor element which comprises: (a) an oxygen ion conductive solid electrolyte body; (b) a target gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a gas to be measured; (c) a reference gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a reference gas; (d) an electrode protective layer provided to cover the target gas electrode, the electrode protective layer having a porosity of 6 to 30% and a thickness of 70 to 500 xcexcm; (e) a catalytic layer provided to cover the electrode protective layer, the catalytic layer being made of heat resisting ceramic grains which hold therein catalytic metal grains whose average grain size is 0.3 to 2.0 xcexcm, a weight of catalytic metal grains per unit area of the catalytic layer, as defined by projecting the target gas electrode on a plane, is 10 to 200 xcexcg/cm2; and (f) a catalytic protective layer provided to cover the catalytic layer.
In the preferred mode of the invention, the catalytic layer has a porosity of 20 to 60% and a thickness of 20 to 150 xcexcm.
The catalytic metal grains may be made from at least one of Pt, Pd, Rh, and Ru.
The electrode protective layer may have a porosity of 6 to 15% and a thickness of 100 to 250 xcexcm.
The electrode protective layer is formed by a heat resisting metallic oxide made of at least one of alumina, alumina.magnesia spinel, and zirconia.
The catalytic protective layer may have a porosity of 30 to 60% and a thickness of 20 to 150 xcexcm.
According to the second aspect of the invention, there is provided an oxygen sensor element which comprises: (a) an oxygen ion conductive solid electrolyte body; (b) a target gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a gas to be measured; (c) a reference gas electrode provided on a surface of the solid electrolyte body so as to be exposed to a reference gas; (d) an electrode protective layer provided to cover the target gas electrode; (e) a catalytic layer provided to cover the electrode protective layer, the catalytic layer being made of heat resisting ceramic grains which hold therein catalytic metal grains whose average grain size is 0.3 to 2.0 xcexcm, a weight of catalytic metal grains per unit area of the catalytic layer, as defined by projecting the target gas electrode on a plane, is 10 to 200 xcexcg/cm2, the heat resisting ceramic grains being each made of Al2O3 which has at least one of a xcex3-phase and a xcex8-phase in crystal structure and to which La2O3 is added, a specific surface of the heat resisting ceramic gains being 50 to 200 m2/g; (f) a catalytic protective layer provided to cover the catalytic layer.
In the preferred mode of the invention, an added quantity of La2O3 is 0.5 to 5 mol % for total 100 mol % of Al2O3 and La2O3.
The catalytic layer has a porosity of 20 to 60% and a thickness of 20 to 150 xcexcm.
The catalytic metal grains may be made from at least one of Pt, Pd, Rh, and Ru.
The electrode protective layer has a porosity of 6 to 15% and a thickness of 100 to 250 xcexcm.
The electrode protective layer is formed by a heat resisting metallic oxide made of at least one of alumina, alumina-magnesia spinel, and zirconia.
The catalytic protective layer has a porosity of 30 to 60% and a thickness of 20 to 150 xcexcm.
According to the third aspect of the invention, there is provided a method of producing an oxygen sensor element which comprises the steps of: (a) preparing an oxygen ion conductive solid electrolyte body on which a target gas electrode is provided so as to be exposed to a gas to be measured and a reference gas electrode provided so as to be exposed to a reference gas; (b) forming an electrode protective layer on the target gas electrode; and (c) forming a catalytic layer on the electrode protective layer by dipping heat resisting ceramic particles in a solution of a catalytic metal grain-forming material to stick catalytic metal salt to the heat resisting ceramic particles, subjecting the heat resisting ceramic particles to heat treatment at 900 to 1200xc2x0 C. to deposit catalytic metal grains on the heat resisting ceramic particles, adding an inorganic binder and a solvent to the heat resisting ceramic particles to produce slurry, applying the slurry to a surface of the electrode protective layer, and subjecting the slurry to heat treatment at 500 to 1000xc2x0 C.
In the preferred mode of the invention, the electrode protective layer is made by plasma-spraying heat resisting metallic oxide particles over the target gas electrode.
The electrode protective layer may alternatively be made by applying an electrode protective layer-forming material containing heat resisting metallic oxide powders to a surface of the target gas electrode and baking the electrode protective layer-forming material.